1. Field of the Invention
The present invention relates to a power steering control apparatus for assisting manipulation or handling of a steering wheel of a motor vehicle on the basis of steering torque information and vehicle speed information. More particularly, the present invention is concerned with a power steering control apparatus in which information available from a so-called anti-skid brake system adapted to control a brake force applied to the motor vehicle in dependence on rotation speed information of wheels of the motor vehicle is effectively made use of.
2. Description of the Related Art
For a better understanding of the present invention, the background techniques thereof will first be described.
FIG. 5 is a block diagram showing a structure of a power steering control apparatus known heretofore. Referring to the figure, the power steering control apparatus is comprised of a power steering controller 20 which is constituted by a microcomputer 4 and which includes an interface 3 through which output signals of various sensors including a torque sensor (indicated only schematically at reference numeral 1) for detecting a steering torque (or angular position) of a steering wheel 1a (also shown only schematically), first and second vehicle speed sensors 2a and 2b for detecting the speed of the motor vehicle relative to ground and others are input to the microcomputer 4, which is so programmed as to calculate a steering assist force on the basis of the steering torque information, which is generated upon steering of a steering wheel and indicative of a steering torque imparted to the steering wheel by a driver of the motor vehicle, and the vehicle speed as input while taking into account the steering direction. For the purpose of ensuring a fail-safe feature for the motor vehicle as described later on, an auxiliary microcomputer 5 is provided for processing the input information supplied via the interface 3 in the similar manner as the main microcomputer 4.
A fail-safe relay 6 is connected to output terminals of the main microcomputer 4 and the auxiliary microcomputer 5 for supplying an electric power from a battery 7 to a motor drive circuit 8 and a clutch actuation circuit 10 in response to control output signals generated by the microcomputers 4 and 5. To this end, the motor drive circuit 8 has an output terminal for supplying a driving signal to a DC motor constituting a part of a steering assist actuator 9 in response to the control signals output from the microcomputers 4 and 5. Further, a clutch driving circuit 10 is connected to output terminals of the main and auxiliary microcomputers 4 and 5 for supplying a driving signal to a clutch actuator 11 in response to the output signals of the main and auxiliary microcomputers 4 and 5.
As is apparent from the above description, the power steering control apparatus shown in FIG. 5 includes two microcomputers 4 and 5, wherein the fail-safe relay 6 is electrically energized to supply the control signal to the DC motor of the steering assist actuator 9 only when the conditions for driving the DC motor as determined by both of the microcomputers 4 and 5 coincide with each other. In other words, only when the results of calculations or arithmetic operations performed by both the microcomputers 4 and 5 coincide with each other, the control signal is issued to the DC motor of the steering assist actuator 9, whereby safety is ensured for the operation of the motor vehicle by virtue of the so-called fail-safe feature.
The power steering control apparatus known heretofore however suffers from the following problems. First, because the steering assist force is determined on the basis of the outputs of the torque sensor 1 and the vehicle speed sensors 2a and 2b, there arises a possibility that the steering force optimal to the road condition can not be determined unless the vehicle speed is detected correctly. In this regard, it should be mentioned that the vehicle speed is generally detected on the basis of rotation speeds of the driving wheels of the motor vehicle. Consequently, when the wheels are locked, for example, upon application of a hard or heavy brake (e.g., emergency brake) in the course of running on a road such as a snow-covered road where slippage is very likely to occur, the motor vehicle will continue to run in spite of stoppage of rotation of the driving wheels (i.e., locking of the driving wheels). In that case, the vehicle speed can no more be detected correctly, which in turn means that the steering assist force is erroneously determined. Another problem can be seen in that when the hard brake is applied in the course of running on a slippery road surface, the steering wheel may assume a so-called floating state under application of a steering assist force determined incorrectly for the reason mentioned above, whereby difficulty is involved in manipulating the steering wheel and hence in maneuvering the motor vehicle.